Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22531−22542
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MoS2‑Decorated Laser-Induced Graphene for a Highly Sensitive, Hysteresis-free, and Reliable Piezoresistive Strain Sensor Ashok Chhetry, Md. Sharifuzzaman, Hyosang Yoon, Sudeep Sharma, Xing Xuan, and Jae Yeong Park* Department of Electronic Engineering, Kwangwoon University, 447-1 Wolgye-dong, Nowon-gu, Seoul 01897, Republic of Korea
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S Supporting Information *
ABSTRACT: Advancement of sensing systems, soft robotics, and point-of-care testing requires the development of highly efficient, scalable, and cost-effective physical sensors with competitive and attractive features such as high sensitivity, reliability, and preferably reversible sensing behaviors. This study reports a highly sensitive and reliable piezoresistive strain sensor fabricated by one-step carbonization of the MoS2-coated polyimide film to obtain MoS2-decorated laserinduced graphene. The resulting three-dimensional porous graphene nanoflakes decorated with MoS2 exhibit stable electrical properties yielding a reliable output for longer strain/release cycles. The sensor demonstrates high sensitivity (i.e., gauge factor, GF ≈1242), is hysteresis-free (∼2.75%), and has a wide working range (up to 37.5%), ultralow detection limit (0.025%), fast relaxation time (∼0.17 s), and a highly stable and reproducible response over multiple test cycles (>12 000) with excellent switching response. Owing to the outstanding performances of the sensor, it is possible to successfully detect various subtle movements ranging from phonation, eye-blinking, and wrist pulse to large human-motion-induced deformations. KEYWORDS: MoS2-decorated laser-induced graphene (MDS-LIG), piezoresistive strain sensor, laser irradiation, crack propagation, subtle strain
1. INTRODUCTION Highly sensitive, flexible, and stretchable strain sensors with low hysteresis and a linear strain response are desirable for the detection of subtle human motion detection, such as wristpulse, phonation, and healthcare applications.1,2 Furthermore, fast responsiveness and stability are other crucial parameters in the accurate monitoring of external stimuli over an extended time. Despite the tremendous progress in the field of nanomaterials, the straightforward synthesis of such sensors in a scalable approach, enabling high sensitivity and stability over a long period, is still challenging for potential applications in wearable electronics, soft robotics, and healthcare monitoring. Various materials such as silver nanowires (AgNWs),3−5 carbon nanotubes (CNTs),6,7 carbon black (CB) particles,8 reduced graphene oxide (rGO),9−11 ionic liquids,12,13 metal nanoparticles,14,15 metal oxides (ZnO and TiO2),16−18 and so forth are typically considered as the active materials for the development of flexible strain sensors. Some of the results are impressive; however, the majority of these studies reports either low gauge factor (GF) or nonlinear response, resulting in low reliability for repeated strain/release cycles. For instance, Yu et al. developed a highly flexible and transparent strain sensor by stacking super-aligned CNT films on a polydimethylsiloxane (PDMS) substrate.19 Although the sensor had a strain limit reaching up to 400%, the resistance of the sensing material lacked reversibility upon strain release. © 2019 American Chemical Society
Generally, at a large-scale strain, the change in resistance is the ultimate result of the complete loss of electrical connection with irreversible degradation; subsequently, high values of GF have been demonstrated.20,21 For such sensors, high sensitivity and linearity under small-scale strain with a reversible electrical connection for repeated strain/release cycles are the main challenges to fulfill their potential applications. Among two-dimensional (2D) nanomaterials, graphene is the most intensively studied because of its excellent mechanical, chemical, and electronic properties.10,11,22 The unique physiochemical properties make graphene a prospective candidate material for applications in supercapacitors,23 electrochemical biosensors,24 and physical sensors.10,22,25,26 Graphene can be produced from various carbon sources such as laser exfoliation of highly ordered pyrolytic graphite27 or graphite oxide23,24 and laser-induced chemical vapor deposition.28 Apart from these methods, direct laser scribing on carbon-rich precursors [polyimide (PI), wood, etc.] under CO2 laser can also generate porous graphene networks.29,30 To date, laser-induced graphene is found to be a robust and rapid method for preparing three-dimensional hierarchical networks of porous graphene on flexible substrates.23,29,31 The technique Received: March 19, 2019 Accepted: June 5, 2019 Published: June 5, 2019 22531
DOI: 10.1021/acsami.9b04915 ACS Appl. Mater. Interfaces 2019, 11, 22531−22542
Research Article
ACS Applied Materials & Interfaces provides a rapid route toward a simple, facile, and scalable approach for patterning variety of carbon precursors that can be converted into amorphous carbon.31 Graphene obtained from this technique has stable electrical performance; however, when used in the strain sensing applications, the repeated number of strain/release processes leads to structural damage of the graphene flakes, thereby increasing the resistance of the film irreversibly. Ren and co-workers32 employed a simple liftoff process to remove the unreduced graphene oxide present in LIG. After laser scribing of graphene oxide, the sheet resistance of the total graphene structure decreased to 700 Ω/sq, improving the GF up to 673. However, the sensor has a nonlinear response with a narrow working range (up to 10%) and poor mechanical durability up to few hundreds of cycles. Recently, Gao et al.33 patterned the features by direct laser scribing on the ecoflex polymer, thereby converting the material into large band gap silicon carbide (SiC) without the need of any post-synthesis. Although the GF achieved was excellent, the device suffered from overshoots in incipient cycles of mechanical durability test and also the device has limited stretchability (12 000 cycles). Finally, the feasibility of the sensor is confirmed with large human body movements to subtle deformations within the human body such as phonation and wrist pulse.
2. EXPERIMENTAL SECTION 2.1. Synthesis of MoS2. A facile hydrothermal method was adopted for the preparation of MoS2 nanosheets.42 All chemical reagents were of analytical grade and used without further treatment. A quantity of 1.1 g of sodium molybdate dihydrate (Na2MoO4·2H2O) and 1.0 g of thiourea (NH2CSNH2) was added to 35 mL of deionized (DI) water and magnetic stirred for about 10 min. The pH value of the composite was adjusted to less than one by adding 12 M HCl. Then, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 200 °C for 24 h. The schematic for the synthesis of MoS2 is illustrated in Figure S1, Supporting Information. Finally, after natural cooling, the black MoS2 was collected by vacuum filtration and washed several times with copious amounts of DI water and then dried at 60 °C for 24 h. The powder of MoS2 (4 mg) was exfoliated by vigorous ultrasonication for 1.30 h at 10 °C in a 10 mL mixer solution of n-methyl-2-pyrrolidone and DI water (1:1 vol ratio) to produce the homogeneous dispersion. 2.2. Fabrication of the MDS-LIG Sensor. A three-dimensional porous network of MDS-LIG was prepared by CO2 infrared laser scribing (Microlaser C40, Coryart, Korea) of a MoS2-coated commercial Kapton PI sheet (200 μm). Laser scribing was performed using a 10.6 μm CO2 laser at fixed dots per inch of 1000 at various powers, gaps, and speeds. The optimized laser parameters with a power of 7.2 W, a gap of 0.075 μm, and speed of 150 mm s−1 (linear laser density of ∼51 J m−1) at ambient conditions were adopted for the fabrication of the final device. To coat a thin film of MoS2 on the polymeric substrate, the as-prepared MoS2 solution (100 μL) was drop cast on a tape-fixed area of 48 × 4 mm and the solvent was evaporated at 120 °C for 30 min. The PDMS prepolymer and its cross-linking agent (Sylgard 184, Sigma-Aldrich) were prepared in 10:1 weight ratio, degassed, and then coated by a doctor blading method (thickness of ∼0.5 mm). After PDMS curing at 85 °C for 2 h, the exterior of MDS-LIG was peeled off from the PI substrate. The electrical contacts were made by connecting the copper tape with the help of silver paste. To obtain the final sensing unit, the transferred patterns were again encapsulated by the PDMS suspension and then cured, followed by cutting and electrode exposure. 2.3. Characterizations. The surface morphology of MDS-LIG was investigated by high-resolution field emission scanning electron microscopy (FESEM, JSM-6700F) images. For the elemental analysis, energy-dispersive X-ray spectroscopy (EDS) was also performed using the same instrument. Raman shifts were obtained by Renishaw (inVia Raman Microscope) using a 514 nm excitation laser. X-ray photoelectron spectroscopy (XPS) spectra were measured using a PHI 5000 VersaProbe (ULVAC PHI, Japan). Fourier transform infrared spectroscopy (FTIR) spectra were recorded in the range 500−4000 cm−1, employing Thermo scientific NICOLETiS10. All of the samples for analytical characterizations were the MDS-LIG films on the PI sheet. A moving fixture (JSV-H1000, Japan Instrumentation System Co., Ltd.) and force gauge (HF-1, Japan Instrumentation System Co., 22532
DOI: 10.1021/acsami.9b04915 ACS Appl. Mater. Interfaces 2019, 11, 22531−22542
Research Article
ACS Applied Materials & Interfaces
Figure 1. Fabrication process and morphology of the MDS-LIG strain sensor. (a) Schematic illustration of the MDS-LIG strain sensor fabrication. The low-power infrared laser irradiation of MoS2-coated PI directly converts the film into well-exfoliated 3D-hierarchical networks of porous graphene decorated with MoS2. (b,c) FESEM images showing the ridge scan line patterns at 0.1 and 0.075 mm, respectively, at the laser power of 7.2 W and raster speed of 150 mm s−1. (d) High-resolution FESEM images of hierarchical porous graphene decorated with laser-irradiated fewlayer MoS2 nanosheets. (e) Photograph showing (i) the successful transfer of MDS-LIG into the PDMS substrate, (ii) twisting, and (iii) the final sensor. The structure is not only stretchable but also mechanically stable which can be twisted and bent without any loss of electrical performance. (f) Chemical structure of two layers MoS2 showing the single layer of molybdenum is sandwiched between two layers of sulfur. Ltd.) in combination with home-made clampers were used to apply longitudinal tensile strain. During the electromechanical characterizations, the cross-sectional area of the sensor in which the tensile force applied was 1.29 mm × 17.62 mm. Instantaneous resistance measurements were performed using an LCR meter (Hioki, IM 3536) at a dc voltage of 1 V. The surface resistance of the LIG porous graphene was measured by a four-point meter (RC3175, EDTM). Optical images were taken by an AcquCAM Pro/U microscope with Olympus, UMPlan FI 10× 0.25 BD lens.
area requires multiple sweeps of laser beams. At low-power densities, the crystalline graphene disperses into the nanocrystalline structure and for power densities higher than the threshold, hydrogenated amorphous carbon atoms liberate on the surface.43 The high-magnification FESEM image of Figure 1d and its inset clearly shows that CO2 laser scribing of the MoS2-coated PI film resulted in a hierarchical porous network of MDS-LIG (Figure S2a, Supporting Information). As seen from the FESEM images, the nanostructure is composed of a wrinkled flake-like morphology, which results from the rapid liberation of carbonaceous and gaseous products.43 A crosssectional view of the MDS-LIG (thickness of ∼47 μm) and PI substrate is depicted in the FESEM image of Figure S2b, Supporting Information. Graphene formation is most likely a photothermal process, in which high localized temperatures of >2500 °C can easily break the C−O, CO, and C−N bonds of the PI polymer.29 When the PDMS suspension cast on the MDS-LIG, PDMS infiltrates into the interconnected channels of a hierarchical porous network of MDS-LIG because of the low surface energy and low viscosity of the PDMS. After curing of the PDMS elastomer, the MDS-LIG pattern was peeled off from the PI substrate. Figure 1e shows the photographs of (i) the replication of MDS-LIG into PDMS, (ii) twisting, and (iii) the final sensor. The MDS-LIG patterns can be easily folded and twisted by any desired angles (Figure S2c, Supporting Information). The schematic of Figure 1f illustrates the
3. RESULTS AND DISCUSSION 3.1. Fabrication of MDS-LIG. The flexible and stretchable strain sensor consists of hierarchical networks of porous MDSLIG produced by direct laser scribing of the MoS2-coated PI film (for details, refer to the Experimental Section) encapsulated within a PDMS elastomer. As depicted in the schematic of Figure 1a, MoS2-coated PI was placed in the focused laser beam of a wavelength of 10.6 μm under ambient conditions. Studies reveal that the fabrication of large-scale porous graphene is possible without the need for an inert atmosphere because of oxidation at the laser-scribed zone. The morphological difference in the carbonized patterns can be observed depending on the scan line gap as depicted by FESEM images of Figure 1b (0.1 mm) and Figure 1c (0.075 mm). The ridges are formed in an orderly fashion along the carbonized direction. As the line width is ∼75 μm, similar to the spot diameter of the laser, carbonization in the targeted 22533
DOI: 10.1021/acsami.9b04915 ACS Appl. Mater. Interfaces 2019, 11, 22531−22542
Research Article
ACS Applied Materials & Interfaces
Figure 2. Performance characteristics of laser-irradiated MoS2 and PI. FESEM images of multilayered MoS2 onto the SiO2/Si substrate (a) before laser irradiation and (b) after laser irradiation. The laser irradiation of multilayered MoS2 down to few-layer MoS2 provides a reliable method to fabricate large-area nanosheets having electrical conductivity comparable to 1T-MoS2. The linear resistance of laser-induced graphene as a function of (c) scribing power, (d) line gap, and (e) raster speed. The scribing power, line gap, and raster speed of 7.2 W, 0.075 mm, and 150 mm s−1, respectively were adopted for the fabrication of the final device. (f) Photographs showing the diminished intensity of LED as a function of tensile strain: (i) 0, (ii) 2.5, (iii) 7.5, and (iv) 15%.
Figure 3. Analytical characterization of the MDS-LIG heterostructure. (a) Raman spectra with and without MoS2 showing the presence of D, G, and 2D peaks. The ratio of peak intensity (ID/IG = 0.81) shows a reasonable number of defects in MDS-LIG. (b) XPS spectra of the MDS-LIG composite showing the different deconvoluted peaks. The C 1s spectrum of XPS indicates the dominant portion of the C−C peak (sp2-carbon atoms) of LIG in the composite. (c) High-resolution XPS spectra of Mo 3d. (d) FTIR spectra comparison between PI, MoS2, and MDS-LIG. The MDS-LIG spectrum shows the absorption of the peaks of PI after laser irradiation.
sheets.34,36 The intralayer transition-metal−chalcogen bonds are predominantly covalent in nature, whereas sandwiched layers are coupled by weak van der Waals forces, such that charge transport occurs between the nanosheets.38 In a typical experiment, the influence of laser irradiation on multilayered MoS2 topography was studied by means of
chemical structure of MoS2 depicting the sandwiched layers. As MoS2 exhibits a hexagonal structure with each monolayer, three stacked layers of S−Mo−S are covalently bonded by weak intermolecular forces. Their common characteristics are strong intralayer covalent bonding among layered structures and weak van der Waals forces between the interlayer 22534
DOI: 10.1021/acsami.9b04915 ACS Appl. Mater. Interfaces 2019, 11, 22531−22542
Research Article
ACS Applied Materials & Interfaces
Figure 4. Electromechanical characterization of the MDS-LIG strain sensor. (a) Linear relationship of I−V curves for the verification of Ohm’s law. The slope of the curve decreases for the strain from 0 to 37.5% (stretching by 15 mm) showing increasing nature of the resistance. (b) Relative resistance change vs strain during stretching (green line) and releasing (red line) up to a safer limit of 25%. The negligible value of hysteresis of ∼2.75% indicates that the change in resistance behavior is fully reversible. (c) Relative change in resistance vs strain comparison between MDS-LIG and LIG strain sensors for the measurement of the sensitivity (i.e., GF). The sensor offers the GF of 236.2 at
